Purpose: :
The directional sensitivity of the retina, known as the Stiles-Crawfordeffect (SCE), originates from the waveguide property of photoreceptors.This effect has been extensively studied in normal and pathologiceyes using highly customized optical instrumentation. Here weinvestigate the utility of a Shack-Hartmann wavefront sensor(SHWS) - an instrument that is commercially available and employedin clinics - as a new approach for measuring the optical SCE.

Methods: :
An existing laboratory SHWS was modified for use in this study.The sensor consisted of a 788 nm beacon that entered the eyewith a 2 mm diameter and that delivered 3 µW. A 17 by17 lenslet array sampled the reflected light across a 6.8 mmpupil at the eye. The SHWS was modified to enable control ofthe beacon entry position (XY) in the pupil of the eye as wellas to include a pupil camera, which provided confirmation ofthe beacon location. Four subjects were recruited, all freeof ocular disease and with normal corrected vision. Pupil dilationwas realized with 1% Tropicamide.Near-IR wavelengths penetratedeeper into retinal tissue and generate more multiply scatteredlight than visible wavelengths, both effects that mask the directionalcontribution of the photoreceptors. To offset these, we tookadvantage of the SH lenslet array to spatially separate thewaveguided light (core of SH spots) from the multiply scatteredlight (side lopes of SH spots). The bright core of each SH spotwas extracted from the acquired frames and used to compose aspatial distribution map of the retinal reflectance in the pupilplane of the eye. Using a non-linear least square algorithm,the reflectance spatial distribution was fit to a five-parametermodel composed of a constant bias and a Gaussian function withdirectional parameter, ρ. Measurements of ρ were acquired at thefovea and retinal eccentricities of 1, 2, and 3 degrees.

Results: :
The average ρ across the four subjects was 0.08, 0.12, 0.17 and0.21 mm-2, starting at the fovea. ρ increases monotonically withretinal eccentricity and is consistent with published valuesafter taking into account the longer wavelength used here.

Conclusions: :
The SHWS has been traditionally employed for measuring wavefrontphase (aberrations) in the eye. Here we demonstrate that intensityinformation can also be extracted from the same measurementsand the spatial distribution of which is consistent with thatproduced by the optical SCE.